Surface-coated cutting tools having single-layer or multi-layer hard coatings of TiC, TiN, TiCN, Al2O3, etc. on the surfaces of substrates made of cemented carbides, cermets, ceramics, etc. have been conventionally used for the cutting of metals, etc. Because such surface-coated cutting tools are used under increasingly severer conditions (for example, those subject to large shock such as heavy, intermittent cutting, etc.), desire is mounting on cutting tools having hard coatings durable to large shock.
JP 2008-87150 A discloses a coated cutting tool comprising a TiN film, a columnar crystal TiCN film having an average grain size (measured in parallel to a substrate surface) of 0.05-0.5 μm, and a peak 2θ of the X-ray diffraction of a (422) plane measured using CuKα rays in a range of 121.5-122.6°, an aluminum oxide film and a TiN film, in this order on a substrate surface. In the columnar crystal TiCN film, C/(C+N) is 0.70-0.90. In JP 2008-87150 A, a starting material gas for the columnar crystal TiCN film comprises a TiCl4 gas, a CH3CN gas, an H2 gas, and a linear hydrocarbon gas having 2-20 carbon atoms excluding CH4, such as C2H6, C2H4 or C3H6. Because the starting material gas contains a linear hydrocarbon gas, the columnar crystal TiCN film has as small an average grain size as 0.05-0.5 μm, and as high a carbon concentration [C/(C+N)] as 0.70-0.90, thereby having high hardness and excellent wear resistance. However, it has been found that because the columnar crystal TiCN film is a single layer having a high carbon concentration, it has insufficient adhesion to the aluminum oxide film.
JP 2005-212047 A discloses a surface-coated member having a fine titanium carbonitride layer, an upper titanium carbonitride layer, and an aluminum oxide layer formed in this order on a substrate by a chemical vapor deposition method. JP 2005-212047 A describes that by using a reaction gas comprising a TiCl4 gas, a CH3CN gas, an N2 gas and an H2 gas to form the titanium carbonitride layer, and by making the proportion of a CH3CN gas higher in the reaction gas for the upper titanium carbonitride layer than in the reaction gas for the fine titanium carbonitride layer, the upper titanium carbonitride layer has larger crystal grain sizes than in the fine titanium carbonitride layer. However, because the method described in JP 2005-212047 A provides the upper titanium carbonitride layer with a higher carbon concentration than that of the lower titanium carbonitride layer, the upper titanium carbonitride layer has insufficient adhesion to the Al2O3 film. Accordingly, the surface-coated tool described in JP 2005-212047 A has a short life.
JP 2005-153098 A discloses a surface-coated cutting tool comprising at least a TiCN film of stripe-shaped crystals grown perpendicularly to the substrate, and an Al2O3 film, which are formed in this order on a substrate made of a hard alloy such as cemented carbide, the TiCN film comprising a carbon-rich TiCN film having a C/N ratio of 1.5-4 and in contact with the Al2O3 film, and a nitrogen-rich TiCN film below the carbon-rich TiCN film and having a C/N ratio of 0.2-0.7, and two or more peaks of the TiCN film existing in a range of a diffraction angle 2θ=60.4-61.5° in an X-ray diffraction pattern. JP 2005-153098 A describes that because the carbon-rich TiCN film exposed after the peeling of the Al2O3 layer has high wear resistance, the tool has stable wear resistance and breakage resistance without suffering rapid wear. JP 2005-153098 A also describes that stripe-shaped crystals in the carbon-rich TiCN film desirably have a larger average crystal width than that of stripe-shaped crystals in the nitrogen-rich TiCN film for higher adhesion in their interface. However, like JP 2005-212047 A, this surface-coated cutting tool comprises the carbon-rich TiCN film on the nitrogen-rich TiCN film, failing to have sufficient adhesion to the Al2O3 layer. In addition, it has poor wear resistance and thus a short life, because of the lack of a layer corresponding to the second layer in the present invention.
JP 2006-315154 A discloses a cutting tool of surface-coated cermet comprising a modified titanium carbonitride layer vapor-deposited on a tool substrate. The modified titanium carbonitride layer is formed by forming a conventional TiCN layer on a thin TiCN film having an average thickness of 0.02-0.5 μm. Because a starting material gas for the thin TiCN film comprises TiCl4, C3H6, N2 and H2, the thin TiCN film corresponds to the first layer in the present invention. Also, the conventional TiCN layer formed by a starting material gas comprising TiCl4, CH3CN, N2 and H2 corresponds to the third layer in the present invention. Because the modified titanium carbonitride layer of JP 2006-315154 A comprises the conventional TiCN layer (corresponding to the third layer) directly formed on the thin TiCN film (corresponding to the first layer) without the second layer in the present invention, it has insufficient hardness and wear resistance. Accordingly, the cutting tool of JP 2006-315154 A has a short life.